[en] Highlights: • Caprolactam (CPL) is employed as Lewis base for one-step anti-solvent process. • 19.2% conversion efficiency of planar perovskite solar cells has been achieved. • Stabilities of perovskite films and devices have been significantly enhanced. • Residual CPL is supposed to well passivate grain boundaries and the interface. Low-cost caprolactam (CPL) has been introduced into one-step anti-solvent process to fabricate efficient and stable planar perovskite solar cells (PSCs) for the first time. It is found that CPL as a Lewis base has the weak coordination ability to Pb(II), and this weak interaction will be easily broken by either anti-solvent dripping or annealing owing to the steric effect, thus leading to a compact film with monolithic grains. Up to 19.2% power conversion efficiency (PCE) has been achieved. Especially, the PSCs exhibit superior stability in ambient condition, which PCE can be retained 95% of its initial PCE after 1200 h. Further investigation suggests that the residual CPL in the perovskite film can well passivate perovskite grain boundaries and the interface by its interaction with Pb²⁺ cations, thus leading to good anti-moisture and thermal stabilities of perovskite films. This work provides a new way to fabricate highly efficient, stable PSCs by adopting an appropriate Lewis base simultaneously for high quality perovskite films and passivation toward grain boundaries and interfaces.